mackguyver, you and I and AlanF have pretty much expressed the same thing in different threads and I haven't changed my opinion on the 300. I also influenced a friend to buy it. However, in my case so far for birds I would almost never go out without 300 X2 and then I typically would end up cropping somewhat and so when I see the 600 prime shots, well, you know ......
Thinking about the 500 and getting to 700 without much degradation is tempting but I'm afraid it would be competition for the 300 and unless my better half was willing to pack it for me I can't see doing what you describe (trekking through the bush) carrying both!
The result would probably be using the 300 for longer hikes and the 500 when stationed closer to a favorite spot and being quite immobile. The gimbal head is really good at times but often it's a pain and I don't pack a tripod on longer hikes unless I know for certain I'll want it. For me stalking is a mix of going where things look hopeful and then often just sitting, since many birds really do get used to one's presence after 15 or 20 minutes and I enjoy reflecting on the scenery and action in a given location. Usually, I don't prefer just walking and walking and walking but I'm at retirement age.
Compromises and more compromises, ugh. It's probably better when you just can't possibly afford it!
Jack